The present invention relates generally to grids for nuclear reactor fuel assemblies and more particularly to an improved bimetallic grid structure which employs a material having a low neutron capture cross section together with a material chosen for its radiation stress relaxation properties.
Nuclear fuel spacer grids are used to precisely maintain the spacing between fuel rods in a nuclear reactor core, prevent rod vibration, and provide lateral support for the fuel rods. Conventional designs of grids for nuclear reactor fuel assemblies include a multiplicity of interleaved straps of egg crate configuration designed to form cells which accept fuel rods (standard cells) and control rod guide thimbles (thimble cells). Each standard cell provides support to one fuel rod at a given axial location through the use of springs and protrusions (called dimples) formed into the metal. A peripheral strap (having the same width as the interleaved straps) encloses the interleaved straps to impart strength and rigidity to the fuel assembly. In order to minimize the lateral displacement of fuel rods and to improve the fuel characteristics of an assembly, a number of grids are spaced along the fuel assembly length. These grids are held in place by bulging or otherwise affixing control rod guide thimble sleeves to the grid.
The existing bimetallic spacer grids employ grid straps, made from a low neutron capture cross-section material, for structural support and use springs mounted thereto, made from a good radiation stress relaxation material, to contact the fuel rods. A "good radiation stress relaxation material" is one which exhibits low stress relaxation during radiation, meaning the material does not suffer a significant loss of resiliency when exposed to intense and prolonged irradiation. The assembly of present bimetallic grids is relatively complicated and time consuming.
Existing unimetallic grid straps which have integral springs are made from a low neutron capture cross-section material. These springs are subject to stress relaxation under irradiation.